This invention relates to an apparatus and method for accurately determining the viscosity of Newtonian and non-Newtonian fluids by means of a controlled needle viscometer. The controlled needle viscometer includes a vertical sample insert tube filled with the liquid of which the viscosity is to be determined. Viscosity of the sample can be determined by measuring the time of fall of a controlled needle through a predetermined distance of the fluid held in a sample insert tube.
A fluid can generally be classified as ideal, Newtonian or non-Newtonian based on its behavior under stress. An ideal fluid has no shear stress in a flow field and its viscosity is zero. No fluids which exhibit this type of behavior in fact exist. In a Newtonian fluid, such as water and glycerol, the shear stress is directly proportional to the shear rate, and its viscosity is independent of the shear rate. In a non-Newtonian fluid, the shear stress is dependent on the shear rate, and its viscosity may vary with the shear rate in a complex manner.
Viscosity is a function of internal friction and of the behavior of a fluid under stress. Therefore, in order to improve the design of pumps, stirrers, mixers, liquid transport devices, and reactors, it is important to be able to accurately determine viscosity. Furthermore, because the molecular weight of a polymer solution is related to its viscosity at zero shear rate, an accurate determination of the zero shear rate viscosity of a polymer solution enables one to obtain an accurate measurement of its molecular weight.
Many methods have been developed to determine the viscosity of fluids. The earliest is the capillary type viscometer, in which a fluid flow is provided through a capillary tube and the drop in pressure across a length of the tube is used to determine the viscosity. This technique suffers from many disadvantages, such as the need for measuring small pressure differences accurately, calibrating the diameter of the capillary tube and keeping the capillary tube clean. Furthermore, it has been found that the capillary tube viscometer is only useful for determining viscosity at high shear rates. It cannot be used to determine viscosity at low shear rates.
Another known technique is falling sphere or falling ball viscometry, first described in G. G. Stokes, Camb. Phil. Trans., 9, p. 8 (1851). In this method the viscosity is determined from the time taken for a sphere to fall through a predetermined distance in an infinite fluid. However, in the falling sphere method, the following assumptions are made: (a) the spheres are falling in an infinite medium, and (b) the density of the falling sphere is in a suitable range for the equation used to determine the viscosity to hold true. Furthermore, the falling sphere must be perfectly round, so that it will fall vertically through the fluid and will not veer in one direction or another or fall erratically.
In practice, spheres can only be made from a limited range of materials, such as, glass, aluminum or steel and their density cannot be adjusted. Further, very few spheres are truly round and, as a consequence, the fall through the fluid medium is often not vertical. Moreover, a fluid must be held in a container. Therefore, the fluid is not, in fact, infinite, and wall effects have to be considered. Thus, inaccuracies arise from the non-vertical fall of a sphere and a correction factor for wall effects must be applied. The falling sphere method does not provide an exact analytical solution for non-Newtonian fluids because of the geometric complexities involved.
Falling cylinder and plunger viscometers have also been designed. See, Lohrentz, et al., A. I. Ch. E. Journal, 6, No. 4, p. 547-549 (1960) and G. S. Smith, J. Inst. Pet., 43, p. 227-230 (1957). Some deficiencies of these viscometers are difficulties in constructing the falling cylinder or plunger; difficulties in obtaining cylinders or plungers with different densities; and difficulties in maintaining a vertical fall through the fluid. To maintain a vertical fall through the fluid, guide pins or bushings are required. Further, the eccentricity effect is very significant. Because of these problems, it is difficult to account for the systematic error in viscosity measurement by the falling cylinder or plunger method.
A rotating cylinder viscometer with two coaxial cylinders, a rotating outside cylinder with a stationary inside cylinder, has been developed to measure the viscosity of non-Newtonian fluids. See Van Wazer et al., Viscosity and Flow Measurement, p. 47-96, Interscience Publishers, New York, 1963. However, the rotating cylinder viscometer is difficult and expensive to make because small torque measurements on the stationary spindle are needed for compensation purposes. Further, it is very difficult to maintain a constant temperature in the system and evaporation of the fluid from the open mouth container is unavoidable. These difficulties are often translated into unacceptably large errors in the viscosity measurement obtained.
Recently, a relatively simple and easily used apparatus and method for the accurate determination of the viscosity of Newtonian and non-Newtonian fluids was developed. That apparatus is the subject of U.S. Pat. No. 4,637,250, issued Jan. 20, 1987, entitled Apparatus and Method For Viscosity Measurements For Newtonian and Non-Newtonian Fluids, to Irvine and Park (the present inventor). The disclosure of that patent is incorporated herein by reference as if set forth in full. The patented apparatus includes a cylinder for holding the fluid for which the viscosity is to be determined; a needle; a needle launcher placed at the top of the cylinder for feeding the needle into the fluid in the cylinder; means at the bottom of the cylinder for collecting the needle; means for maintaining the cylinder, the sample insert tube, and its contents at a constant temperature; and means for measuring the time of fall of the needle between two marks on the wall of the cylinder spaced a predetermined distance. The needle is capable of being adjusted in density, and viscosity is measured by allowing the needle to fall through the liquid in the sample insert tube while maintaining the sample insert tube and its contents at a constant temperature. The time of fall of the needle between the spaced marks on the cylinder (or between transducers) is measured. From this measurement and the dimensions of the apparatus, the viscosity can be calculated.
It is an object of this invention to provide an inexpensive and easily used apparatus and method to determine accurately the viscosity of Newtonian and non-Newtonian fluids over a wide range of viscosities.